Nonassociative learning: Learning involving exposure usually to a single event, and that is presumed not to reflect learning of a relationship between multiple events.

Habituation – A decline in responsiveness to repeated stimulation arising from a central change in the organism.

Some Characteristics:

1. Habituation is stimulus specific. Habituation training on one stimulus does not generalize to other stimuli unless the test stimuli are perceptually similar to the first.
2. It shows spontaneous recovery when the stimulus that has undergone habituation is tested after a rest.
3. Habituation is not motor fatigue or sensory adaptation. Stimulus specific habituation shows how a decline in responsiveness cannot be due to motor fatigue, however, the phenomenon of dishabituation shows how a decline in responsiveness cannot be due to either motor fatigue or sensory adaptation. Therefore, the decline in responsiveness that we call habituation must have been due to a change in the central processes that intervene between sensory and motor neurons (this is usually thought to occur in the central nervous system). In other words, habituation refers to the reduced processing of information that accompanies repeated stimulation in the circuit that relates sensory input to motor output.
4. Habituation is sensitive to the ISI (inter-stimulus-interval). Davis (1970) demonstrated that short ISIs are better than long ISIs at promoting short term habituation, but that long ISIs are better than short ISIs at promoting long term habituation. This experiment nicely demonstrates a distinction between two types of habituation (short and long term).

Dishabituation – The recovery of responsiveness to a stimulus that has undergone habituation training due to the recent occurrence of an extraneous stimulus.

Sensitization – The increase in responsiveness to a stimulus that has not undergone habituation training thought to arise from a general arousal process.

1. The potentiated startle response procedure nicely illustrates the concept. In this situation, startle responding to the 1^st presentation of an auditory stimulus (e.g., tone) is assessed in two groups of rats. In the first group, the control group, the baseline level of responding is determined by simply presenting the tone to the rats and measuring the startle response. In the second group, experimental group, startle responding to the tone is assessed some time after presentation of some sensitizing event (like electric foot shock). The startle response in this group is increased relative to that seen in the control group.

2. Whitlow (1975) demonstrated that it is possible for an extraneous event to dishabituate, but not sensitize, responding to another stimulus. On these grounds, he inferred that dishabituation and sensitization, although both referring to increases in responding, have separate underlying mechanisms. If the two phenomena had a common underlying mechanisms, then whenever a stimulus could serve as a dishabituator, it should also act as a sensitizer (and visa versa). His experiment showed that this was not the case.